It appeals to an obscure and perhaps quixotic tome that seems to be an exercise in armchair etymology that I would appreciate your having a peek at as I think it is misinformation but I lack the skills to be sure or to correct it.

My thought is that it is wrong but if it happens to be right it would be significant so I don't want to get it be mistaken.

Is "asteios" derived from "aster" ("star") or from "astu" ("city")? What can we be sure of from the grammar? My helps all say it is from "astu" but since that is being challenged I wonder if the grammar can settle the question of etymology with some certainty?

I don't think an appeal to grammar will help all that much. There are six uses in Rahlfs LXX: Ex. 2:2; Num. 22:32; Judg. 3:17; Judith 11:23; 2 Mac. 6:23; Sus. 1:7, plus a further two in the NT referring to the Moses case. In all, the grammar is basically that of an attributive or a predicate adjective, with or without a copula verb. The head nouns though are either men or women or a way of life or a thought/consideration.

The stackexchange post you mention quotes a text which offers no reasoning for it's chosen etymology. The argument that ἀστυ is not used is not very convincing as a reason to discount it in my view. There are no instances of adjectival forms of πολις in the NT and only two of quite different form, coming from πολις and another root, in the LXX. The argument that they would have used πολιτης doesn't make sense to me as that word is a noun also. And without getting into the whole NT quoting the LXX thing, if indeed it is a quote why would it not use the LXX wording ?

BDAG argues from the city is beautiful/refined, country is rough and ready contrast which is easily understandable. It also makes this more an idiomatic or slang type thing, which again makes sense to me to be honest.

Of course, none of what I've written above actually decides the point, but I'm not convinced by the stackexchange post's arguments.

It might be interesting to check and see if the word appears in any Greek etymological dictionary, which I don't have alas.

Is "asteios" derived from "aster" ("star") or from "astu" ("city")? What can we be sure of from the grammar? My helps all say it is from "astu" but since that is being challenged I wonder if the grammar can settle the question of etymology with some certainty?

According to Beekes, ἀστεῖος is derived from ἄστυ. I can't see how it can be derived from ἀστήρ--you have to account for the ρ.

Is "asteios" derived from "aster" ("star") or from "astu" ("city")? What can we be sure of from the grammar? My helps all say it is from "astu" but since that is being challenged I wonder if the grammar can settle the question of etymology with some certainty?

According to Beekes, ἀστεῖος is derived from ἄστυ. I can't see how it can be derived from ἀστήρ--you have to account for the ρ.

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Stack exchange is largely a crap shoot. The individual's "criterion" for his belief that it derives from ἀστήρ simply shows that he is ignorant of linguistics.

Actually, the cited text claims that asteios derives "from aster, which means "star", not from asteios which means "city"". This suggests that the author of the cited text believes that "asteios" means "city" showing that he does not understand that asteios is already declined, leaving me with no confidence in his qualifications right off the bat.

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Stack exchange is largely a crap shoot. The individual's "criterion" for his belief that it derives from ἀστήρ simply shows that he is ignorant of linguistics.

Actually, the cited text claims that asteios derives "from aster, which means "star", not from asteios which means "city"". This suggests that the author of the cited text believes that "asteios" means "city" showing that he does not understand that asteios is already declined, leaving me with no confidence in his qualifications right off the bat.

Good catch. It used to be such people only annoyed and/or influenced a small circle of acquaintances. Now, thanks to modern technology, they can annoy thousands or more.